development of a vision system for ship hull ... - ePrints@USM

image the hull structure with greater detail and at far less expense than in a traditionaldry dock setting.ROV aided dam inspection developed by Batlle et al. (2003) reported that theyhad a problem with the thrusters configuration. In this instant, the ROV cannot movelaterally (heave movement) while keeping a constant heading. This turned out to be aproblem when trying to inspect the dam while keeping a constant distance to it. Somesolutions are taken into account in order to solve this drawback. This includes changingthe configuration of the thrusters or adding two more thrusters to the vehicle. Anotherimportant improvement will be the addition of forward-oriented sonar. In this way, theROV can position itself at a constant distance with respect to the wall. Thruster’sconfiguration and pan-tilt camera are also important in this application besides the laserbeam technique itself.2.4 Underwater Laser Triangulation techniquesThere a few criteria have to be taken into account in order to get the bestmeasurement result, namely, laser device selection, hardware manipulation andfeature extraction algorithm.2.4.1 The characteristic of laser light over the undersea turbidityThe penetration of laser light and its characteristic over the undersea turbidityhas been developed by Hoshino et al. (2004). They found that, the effect of wavelengthof the light on the attenuation characteristic is huge and only the blue light (650nmwavelength) can penetrates the water up to the depth of 55m. The extinction coefficientincreases in proportion to the increase of the turbidity. It becomes possible that, thelaser device is also suitable for the undersea measurement system. Furthermore, thepermeability condition does not change by the output of the laser and that the power ofthe penetrating laser in turbid water attenuates exponentially.20